@article{DE_1965_1_2_a4,
     author = {V. A. Lunkevich and K. S. Sibirskii},
     title = {On conditions for a center},
     journal = {Differencialʹnye uravneni\^a},
     pages = {176--181},
     publisher = {mathdoc},
     volume = {1},
     number = {2},
     year = {1965},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1965_1_2_a4/}
}